12: How to size a heat exchanger for a swimming pool?
Selecting the correct heat exchanger is very important to ensure the pool heats up quickly to desired temperature. The main issues to consider when sizing a swimming pool heat exchanger are;
- Pool size – what is the water capacity? Heat exchangers are sized according to capacity, so a unit designed to heat a 80m³ (18,000 gal) pool would be no use if you have an 180m³ (39,500 gal) pool.
- How is it heated? Usually the choice is either a boiler or renewable energy. If it’s renewable energy, select a heat exchanger specially designed for the lower temperature water provided by solar panels or heat pumps, as these units need less energy to heat the pool to the required temperature.
- Boiler water temperature – however, most pools will be heated by boilers, so what is the temperature of the boiler water? Usually it’s between 80°C and 85°C – the ideal temperature for pool heating. Some boilers are lower – around 60°C. So, using 82°C water, a heat exchanger providing 110kW should heat your 180m³ pool efficiently. But if the boiler water temperature is only 60°C, the heat available to transfer drops to around 60kW – a reduction of over 40%, so a larger heat exchanger would be required for the pool to achieve full temperature.
- What are the water flow rates? Flow rates are vital for the heat exchanger to transfer thermal energy to the pool. If the hot water flow rate is too low, the available energy will not be passed through the heat exchanger. However, the flow rate of the pool water is equally important. People often think it is important to generate a large temperature differential between the pool water entering and leaving the heat exchanger. They are happy if the pipework connected to the outlet of the heat exchanger is noticeably warmer than it is at the inlet.
In reality, this actually reduces the efficiency of the heat transfer process! This is because the pool water flow is too low – the water remains in the heat exchanger for too long, so a much smaller volume of water is being heated to a slightly higher temperature. However, with higher flow rates, the time taken to turn over the pool water will reduce and even a small increase in the temperature of the pool water through the heat exchanger (1.5° C for example) will have a greater effect on the heating efficiency of the pool.